Class D audio amplifiers are well-known and widely recognized to provide energy efficient audio drive of a loudspeaker load by switching a modulated audio signal e.g. pulse width modulated (PWM) or pulse density modulated (PDM), across the loudspeaker load. Class D audio amplifiers typically comprise an H-bridge driver with a pair of output terminals coupled to respective sides or terminals of the loudspeaker load to apply an oppositely phased pulse width modulated or pulse density modulated audio signals across the loudspeaker. Several modulation schemes for pulse width modulated audio signals have been utilized in prior art PWM based class D amplifiers. In so-called AD modulation, the pulse width modulated audio signal at each output terminal or node of the H-bridge is switched between, or toggles between, two different levels in opposite phase. The two different levels typically correspond to the upper and lower power supply rails, respectively, such as the positive and negative DC voltage supplies of the output stage. In so-called BD modulation, the pulse width modulated signal across the loudspeaker load is alternatingly switched between three levels of which two levels correspond to the above-mentioned upper and lower DC power supplies while the third level is zero that is obtained by simultaneously pulling both terminals of the loudspeaker load to one of the DC power supply rails. In multi-level PWM modulation as described in the applicant's co-pending patent application PCT/EP2011/068873, a third supply voltage level, often a set to a mid-supply level between the positive and negative DC supply rails, is applied to output node(s) of the output driver such that for example a 3-level or 5-level pulse width modulated signal can be applied across the loudspeaker load by an appropriately configured output driver.
However, there is a continued need in the art for reducing the power consumption of class D audio amplifiers, in particular at small audio output levels and quiescent operation, to prolong battery lifetime of portable audio equipment, reduce heat dissipation etc. At small output signal levels, the capacitive switching losses of the output transistors of the output or power stage can represent a significant portion of the total power consumption of the class D audio amplifiers and therefore render the overall power efficiency sub-optimal under these operating conditions.